This invention relates, in general, to data communication and, in particular, to controlling the flow of information between senders of information and receivers of that information, when the information is being forwarded across links being used as channels.
Communication between computer systems usually involves a sending system (sender) sending a command to a receiving system (receiver) over a link used to couple the sender and the receiver. The receiving system then, typically, sends a response back over the link to the sending system.
One example of a link used by International Business Machines Corporation to communicate between a sender and a receiver is an intersystem channel link. In particular, an intersystem channel link couples an intersystem channel adapter on one system (e.g., a central processor) that may be used for sending or receiving messages with an intersystem channel adapter on another system (e.g., a coupling facility that contains data shared by any central processor coupled thereto) that may also be used for sending or receiving messages.
An intersystem channel link supports connections up to approximately 20 kilometers, and the propagation time on the link (at 20 kilometers), when using fiber optic conductors, is approximately 100 microseconds in each direction. To minimize message latency, a minimum number of round trip acknowledgements is used, and thus, large data buffers at each end of the link are needed for flow control. In one example, a message exchange requires only one single round trip over the link.
As more and more data is desired to be included with the message, larger data buffers are needed to maintain the single round trip flow control. However, eventually, the expense of large buffers becomes prohibitive and the flow control is modified to add intermediate acknowledgement exchanges to throttle the data transfer preventing the buffers from overrunning. These extra exchanges, though, significantly increase message latency.
Based on the foregoing, a need exists for a more direct communication link between senders and receivers of information. Further, a need exists for a communication link that does not need to couple channel adapters, but still may act as a channel. That is, a need exists for a communication protocol in which the advantages of a channel (e.g., having a direct memory adapter engine and offering protection of memory) may be realized. Additionally, a need exists for a communication protocol that does not require large data buffers and does not significantly add to message latency.
The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a system of controlling the flow of information across links between senders and receivers of data. The system includes, for instance, a packet including a sequence number usable in maintaining delivery order of the packet, the packet having no memory address and requiring no explicit individual response; means for sending the packet from a sender to a receiver across a link; and means for using the sequence number to determine if the packet is in proper order for processing by the receiver.
Advantageously, the flow control capabilities of the present invention provide a more direct communication path between senders and receivers. Further, the flow control protocol does not require large data buffers and offers low latency messaging. Thus, the present invention is less expensive than other communication protocols.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.